This invention relates to a process for the manufacture of an abrasive bonded to a flexible support using aqueous mixtures of a resol and a polyvinyl alcohol as binders.
Abrasives bonded to a flexible support, such as, for example, emery paper, abrasive cloth, abrasives with combinations of cloth and paper and abrasive fibres, are usually manufactured in such a way that the support material is in the form of a web approximately 1 to 1.5 m wide and up to several kilometers long, and are provided on one side with a thin film of a liquid binder, known as the basic binder. The abrasive particles are scattered in this basic binder film, generally electrostatically whereby the abrasive particles are in a desired orientation with their longitudinal axes at right angles to the surface of the support material. After the particles have been scattered, the resulting intermediate product is subjected to a heat treatment, by passing through a heating duct, whereby the binder is dried or cured. In general, the binder must be solidified to the extent that the abrasive particles cannot be displaced or broken away during any further treatment. Depending on the particle size, upon which the layer thickness of the binder depends, this heat treatment generally, lasts from 0.5 to 3 hours, during which the temperature is raised from about 20.degree. C. to 110.degree.-120.degree. C. in definite steps. Then a second stronger binder layer, known as sizer, is applied, which generally contains calcium carbonate filler, and which is then similarly heat treated in a heating duct at a temperature of 20.degree. C. rising to 120.degree.-130.degree. C. for 3 to 6 hours until curing is effected.
In order to achieve high production with such long heat treatment times, the heating ducts have to be designed in the form of suspended ducts (heated with circulating hot air), such as loop or festoon dryers, which have lengths up to 100 m for a pre-drying hanging zone (intermediate hanging zone) and up to several hundred meters for the main hanging zone. Investment costs for suspended ducts are very high due to their expensive construction and the size of the installations. Despite the high outlay, loop dryers have however, a series of inherent defects which considerably complicate or render impossible the maintenance of a consistent quality for the products.
To maintain a uniform temperature distribution, the particular dimensions of the suspended ducts produces a problem which can be only inadequately solved despite refined air-circulation and heating techniques. Increasing the blowing rate of the hot air should, in principal, reduce the drop in temperature to a manageable degree, but, in practice, is limited by the relatively unstable suspension of the web. Varying degrees of curing over short portions of and a partially blistered structure of the binder layers are the frequent result of the drop in temperature which unavoidably occurs. These undesired effects can be minimised by a careful increase in temperature, although this leads, in turn, to lengthening of the process and, consequently, an extension of the ducting installations.
Quality is also reduced by the effect of the vertical arrangement of the abrasive material in the loop dryer. If the synthetic resin binder is liquified too greatly by a more rapid increase in temperature, it tends to flow and the originally perpendicular abrasive particles turn over and/or are displaced thereby causing disadvantages in respect of the abrasion characteristics of the product. Additionally, the more or less sharp bends at the tops or bottoms of the loops give rise to potential faults or fractures after subsequent stretching of the web.
Horizontal drying units do not present these disadvantages so that such units are highly suitable for the processing of quick-drying or quick-gelling binders, e.g. hide-glue or urea-formaldehyde resins. Hitherto, it has not been possible to process aqueous-liquid resols in horizontal drying units with an economically acceptable throughput rate, due to the long drying times required, so that horizontal drying ducts of 400 to 500 m in length have been required for a continuous drying operation.
It has been proposed to shorten the long drying processes required for phenolic resin binders used in the manufacture of flexible abrasives in loop drying installations by adding polyvinyl alcohol (PVAL) in aqueous solution or cellulose derivatives, starch or starch derivatives, to a phenolic resin capable of dilution in water. Although it has been possible to partially shorten the drying times, the temperatures used were not more than 100.degree. C. If the proportion of any additives present becomes too large or if the solids content has to be sharply reduced owing to the excessive viscosity of the binder, abrasives of low quality are obtained.
Tests have shown that attempts to shorten drying times of phenolic resin binders down to acceptable times of less than 5 minutes by raising the drying temperatures to 110.degree.-130.degree. C. have failed, as the phenolic resins suffered severe blistering and insufficient binding of the abrasive particles to the support was achieved.
These disadvantages show that an improved method is required to effect the process of drying and curing abrasives bonded in phenolic resins. It is also desirable to avoid the use of a loop dryer which is uneconomical with regard to capital expenditure and operation in such processes.